Below is a creative essay I wrote, inspired by my scuba diving experiences, discussing the fate of coral reefs in our changing world.
The sun hid behind grey, menacing clouds; rain was coming, though I could tell, not for a while. A cool, fast breeze whirled past me, and I lurched back and forth with the rise and fall of every wave. Cold water splashed over my neck and up my jaw. I fought against the force of the ocean, trying to stay close to my group. Holding tightly to the yellow rope that extended from the shore down into the water, I was glad for the gardening gloves that kept my hands protected against the rough nylon.
The instructor shouted commands at us in rapid Mandarin. I could hear the voices of other students behind me, the lilting rise and fall of laughter, shrieks as swells of water pushed them around the cove. The colossal crash of falling waves played on loop like a backing track to the more prominent sounds of birds, people, and wind in my ears. I watched the instructor as he explained again how to use our buoyancy controls, occasionally losing focus to wipe salty water away from my eyes and mouth.
“Li Na!” He shouted over the wind and the waves. “Do you understand?”
“I understand!” Well, maybe 70 percent.
Satisfied, he descended beneath the surface as I fumbled to clamp my teeth around the snorkel. Pulling on my buoyancy controller, I followed him. Cold water rose above my chin, my mouth, and over the plastic of my suctioned-on goggles.
Water filled my ears, and a muffled stillness settled over my senses. Without the stark contrast of the open air, the sea water didn’t feel nearly as cold. The wind and the waves, the students laughing above us only moments before seemed like part of another world. I felt my goggles compress against the cheeks and chided myself to remember to breathe in through my mouth.
I exhaled. My breathing contraption gave a subtle hiss, followed by an overlapping blub-blub-blub that sounded like bubbles. I knew oxygen was running from the tank up into the snorkel and into my mouth. Other than the hiss and blub echoing my breaths, there were no sounds at all.
I looked ahead. The shallow sea surrounded me, a yellow nylon rope leading down in to the dark abyss. My instructor was already ten yards ahead. Quickly, I pulled on the rope, flapped my flippered feet, and swam down after him.
The nylon rope took us deeper, and I swallowed hard to release the pressure building up around my ears. I re-adjusted my buoyancy, struggling to stay neutral, level with the seafloor.
Finally, the instructor stopped and waited for me to catch up. He flashed me a thumbs up, which I returned enthusiastically. All good. Then he gestured around me, and I realized I had been so concerned with breathing right, staying balanced, and not falling behind that I had forgotten to lock around.
There were corals all around me, coating the seafloor and the boulders beside me. They filled my eyes in muted shades of orange, purple, and brown. As we swam further, flashes of colourful fish zigzagged in and out of our path. I saw fish with deep blues, black and white stripes, neon yellows, oranges, and purples, fish as small as my little finger and others longer than my forearm, some wide and round, some skinny and slender. The sheer extent of their diversity took my breath away. That all this life could exist in one space, in one ecosystem, astounded me.
We swam deeper, through alleyways in the rock, up and around tunnels and caves. Drifting along the seafloor reminded me of walking down the streets of Toronto; having lived in small towns for much of my adult life, whenever I visit a city, the sky-high buildings and their formidable architecture takes my breath away. Under the water, the towering expanses of reef formed a natural city infinitely more breathtaking than anything man could create.
Coral reefs are one of Earth’s most beautiful creations, home to 25% of all marine life, even though they cover just 1% of the ocean’s floor. They belong to a group of primitive animals called cnidarians, named for the stinging tentacles they use to capture prey. However, the corals that build reefs, called stony corals, are distinct in that they secrete a hard skeleton made of limestone (calcium carbonate). The body of a stony coral, called a polyp, is comprised of two layers; an external ectodermis, which functions like skin, and the internal gastrodermis, which contains digestive tissues. Their tentacles extend from the ectodermal layer of the polyp, each loaded with stinging cells called nematocysts.
Below the polyp lies a cup-shaped, hard skeleton made of calcium carbonate, called the corallite. During the daytime, the coral retracts its polyp into the corallite to protect itself from predators. At night, the polyp lifts itself out of the corallite, and makes use of its stinging tentacles to catch prey. When corals die, their skeleton remains behind as limestone rock. The limestone rock that forms cliffs, crevasses, and boulders today is thanks to corals from millions of years ago.
How did such a unique animal come to be?
Around 740 million years ago, free-swimming coral larvae attached themselves to submerged rocks growing along the edges of continents and islands. After the most catastrophic mass extinction of the 5 recorded in Earth’s fossil history (through which corals survived) an unusual change occurred; reef-building corals were infected by a single-celled planktonic (free-living) organism known as zooxanthellae. In nature, an interaction between two organisms that provide food or other benefits to each other is called a symbiotic relationship. The partnership benefitted both species, and over time most coral became hosts to these small plant organisms, beginning the symbiotic relationship that led to the formation of the reefs as we know them today.
Within the gastrodermis layer of corals’ polyp live millions of zooxanthellae. Zooxanthellae are primary producers, meaning they produce their own energy from non-living sources, and are the source through which all other organisms obtain energy. Primary consumers like herbivores eat primary producers, and carnivores eat the herbivores, all the way up the food chain. Zooxanthellae produce energy through photosynthesis. They take in sunlight, carbon dioxide, and water and convert it into food for themselves and their coral hosts, converting carbon into a form that corals can use to build their calcium carbonate skeletons. In exchange, corals employ a process called cellular respiration, using oxygen to break down molecules into chemical energy and providing zooxanthellae with the byproducts of the reaction, carbon dioxide and water.
However, even with carbon dioxide and water, the most crucial component of photosynthesis is still missing – sunlight. To ensure that zooxanthellae continue to get this essential ingredient, corals must build their corallite up towards the surface of the water, towards the sunlight. Corals secrete calcium carbonate from their tissues to build onto their skeleton, their polyps reaching out towards the sunlight so that zooxanthellae can continue to photosynthesize and provide them with food and energy.
Throughout Earth’s history, as sea levels rose, corals were constantly building onto their skeletons, growing up towards the surface of the water. The towering structures that rise like skyscrapers beneath the surface of the water are the result of millions of years of corals hard at work, secreting calcium carbonate, stretching their polyps up towards the sky.
The dynamic landscape that these corals provide creates a home for more than 4 000 species of fish; within the cracks and crevices between each coral in the reef, marine species thrive, finding safe spaces to rest, mate, and lay eggs. Essential nutrients and food are limited in the vast, open ocean, and corals offer marine species access to food as well. Zooxanthellae produce so much carbon from photosynthesis that not even the corals can eat all of it; excess carbon is excreted as a slimy algal mucus on the external ectodermis that covers the coral and protect it from disease or from drying out during low tide. Carbon-loving bacteria find the algae and consume it, growing in number and attracting small critters like crabs, shrimp, and snails, which in turn eat the bacteria. Then, fish come to the reef, lured by the promise of a feast on small organisms. Some fish even enjoy feasting on the algae themselves. Organisms at each trophic level, which are hierarchical levels at represent groups of organisms with similar functions in an ecosystem, are drawn to the reef by the promise of food.
Fish aren’t the only ones enticed by the lure of food; coral reefs are also a source of food and income for over 500 million people. Healthy coral reefs contribute to fishing efforts and tourism, such as scuba diving and snorkeling sites like the one I visited. Coral reefs also act as barriers that reduce wave energy and protect coastal areas from tsunamis, preserving people’s homes and communities.
On my journey through the reefs, protected by a neoprene suit and an oxygen tank, the remarkable animals that live there created a kaleidoscopic wonderland in front of my eyes. White-scaled, orange-striped, triangular-shaped fish swam past my goggles, their twisting bodies catching the falling rays of light and sending glimmers across their scales. Their long, pointy nose gave them the appearance of an arrowhead, their small white tail like the shaft extending from it. True to their imagery, they darted through the water in short little bursts, like arrows shot from a bow. These vibrant fish are among the most distinct and prominent of those fish that frequent coral reefs. As foraging predators, butterfly fish search the reef for food, eating small sponges and worms and even the algae on the polyp of the coral themselves. The abundance of butterfly fish on coral reefs helps them serve as a bioindicator of reef health – the state of the reef can be inferred based on their presence or absence.
Distracted, I looked down, and saw a long, winding, tube-like thing sitting on the bottom of the ocean. A sea snake! Patterned with white and electric blue stripes, the way the snake wiggled through the water looked alien to me. I watched it swim away, towards the inner depths of the reef that it called home, no doubt looking for its next meal. Sea snakes eat small fish, prawns, and mollusks that live in the reef, producing venom that paralyzes their prey. However, their venom is weak, and when a predator comes into sight, sea snakes make use of the many cracks and crevices in the reef to hide.
In the distance, a great, big, circular mass swam by. As it got closer, I recognized the thing to be a sea turtle, and my heart leapt – I never imagined I would get to see a sea turtle in its natural habitat, here, in the ocean, not on the screen playing Crush in Finding Nemo. The sea turtle, probably confused by the three large, black-clothed people staring at it, swam away. Perhaps it was saying goodbye to the reef that it once called home; sea turtles also depend on coral reefs for protection and shelter in their juvenile stage, growing up safe from predators within pockets of the reef. Adult sea turtles eat the excess algae that grows on outside of the coral polyps, benefiting both species by reducing the buildup of algae for corals and providing the turtles with sustenance.
All these creatures and so many more depend on coral reefs for survival. It is their hotspot for food, shelter, and mating. At some point in their life cycle, they live, feed, or reproduce in the reefs. Coral reefs house a diversity of life unlike anywhere else on Earth. The ecosystem they create is so efficient, so productive, and so stable, that the life that evolved there has had millennia to branch apart into a thousand unique species.
As I continued swimming, the scenery began to change; gone were the colourful flashes of swimming fish. Gone were the muted orange, purple and green bodies of corals; instead, I saw white, empty branches, ghostly shells of the life that once thrived. Not a single fish swam by. The water held an eerie stillness, a dichotomic reminder of everything moving in the thriving reef I had just left. The crowded community of life behind me extended into a vast underwater wasteland.
What had happened here? What could have caused these corals to turn a stark, bleached white?
To find the answer we must look back in time, searching for knowledge from over a century ago, when a young marine scientist first asked that very question. In the 1920s, Englishman and marine biologist Maurice Yonge undertook a year-long expedition to Australia to study the Great Barrier Reef. The expedition, funded by the Royal Geographical Society of Australasia, comprised of twenty established professionals. Since under-water diving apparatus had yet to be invented, the team of researchers used a diving bell and a car tire pump to explore the reef underwater. This dangerous and difficult method wasn’t easy to set up or efficient to use, so much of the work observing coral relied on times of low tide when one could wade amongst them.
Maurice spent many hours, spanning many months, wandering between the corals during low tides, noting in his observations that the water “was literally hot to the touch”.
A month later, he walked amongst the same reef during low tide, and saw “great numbers of whitened skeletons of corals, killed by the heat a month previously”. When he looked closer, he realized that many of the corals weren’t dead; they were white and skeletal but living nonetheless. He recalled that in one of his and a colleague’s experiments, corals that had been starved of light or subjected to increased water salinity had bleached in a similar way; white and dead-looking, but deceptively alive. In those experiments, the corals had expelled their zooxanthellae counterparts when the salinity of the water changed or their access to light was restricted. However, Maurice’s observation was the first instance of bleaching caused by high water temperatures.
Maurice took samples from the bleached but living corals and marked their place in the water with a steel tube. A month later, he returned to inspect them. To his surprise, the corals he marked had recovered; their white skeletal shells had changed back to brown, though a paler brown than a typical coral.
Perplexed, Maurice took more samples from the corals. Back in the lab, he analyzed both samples under the microscope. In the first sample, taken when the corals were white after the heatwave, he could visibly see zooxanthellae being expelled from the coral’s tissues. In the second, taken when the corals were once again a healthy brown colour, zooxanthellae were as numerous as they would be in an unaffected, never-bleached coral.
Maurice went on to publish his findings; his evidence showed that under natural conditions, “corals may not only be killed by high temperatures, but … may themselves survive although their contained zooxanthellae have been almost completely ejected.” Yonge’s work in 1921 was the first observed instance of coral bleaching due to high temperatures.
Nowadays, thanks to Maurice, his team, and thousands of marine scientists that came after him, we know much more about coral bleaching. Corals and their zooxanthellae symbionts evolved to thrive within a narrow temperature range. When the water warms to temperatures outside the optimal range, light and heat stress damage the zooxanthellae’s photosynthetic machinery, affecting their ability to photosynthesize. The damaged machinery leads to the production of highly reactive unstable chemicals called reactive oxygen species. To protect themselves from these highly toxic chemicals, corals must expel the damaged zooxanthellae from their polyp tissues.
Since zooxanthellae produce the photosynthetic pigment called chlorophyll that gives corals their colorful appearance, when zooxanthellae are expelled from their tissues, corals turn completely white, as if they were bleached, hence the name ‘coral bleaching’.
Almost a century after Maurice made his discoveries, 2 esteemed marine biologists from the 21st century, Ove Hoegh-Guldberg and Sophie Dove, went back to the same location to observe the same corals. They found that the average water temperature had increased by 1.1 °C, from 26.6°C in 1921 to 27.7 °C in 2019. The sea level rose by twenty centimeters and the reef, ravaged by natural disasters, attacked by coral-eating starfish, and decimated by repeated mass bleaching events, was unrecognizable.
In 1921, the location had been dominated by hard, reef-building corals that provided a dynamic habitat for a variety of marine organisms. Today, the area is full of soft corals, a type of cnidarian that doesn’t build tall, intricate branching structures and offers little protection for sea creatures looking to hide from predators. Among the few hard corals remaining, the scientists noted a shift towards more thermally tolerant boulder-type corals. The structurally complex, branching, and dynamic stony corals were a rarity.
Though they searched desperately, these modern-day scientists found no trace of the many species of sea urchins, snails, and sea stars that Maurice and his team from the Royal Geographical Society recorded in their observations.
Maurice Yonge passed away in 1986, at the age of 87. If he were still alive, would he recognize the landscape of the Great Barrier Reef, where he lived and worked for a year as a young scientist, the way it looks today?
Coral reefs can recover from short-term high temperatures, as Maurice observed, relaxing when the thermostat goes down and letting the small number of remaining zooxanthellae repopulate their tissues. However, if the warm period lasts too long, without the zooxanthellae to support their metabolic processes, corals are left to starve and die. Coral bleaching is what had happened to the white, barren, dead corals I swam past while scuba diving. Coral bleaching events have been recorded for over a century, leading to devastating losses of habitat and reefs worldwide, and the frequency of this phenomenon is only increasing as global temperatures rise.
It is a well-known fact that humans are artificially raising the Earth’s temperature through our emission of heat-trapping greenhouse gases; this is called anthropogenically-induced (human-caused) climate change. Since the Industrial Revolution, when we started burning fossil fuels (such as coal, petroleum, natural gas, tar, and oils) to convert the chemical energy they store into heat and electricity, the global average temperature has risen by 1.1°C. Burning fossil fuels releases large amounts of greenhouse gases, such as carbon dioxide, into the atmosphere. These gases get trapped in the atmosphere and contribute to the greenhouse effect that keeps Earth warm.
When the sun’s rays hit the Earth, they are reflected back out towards space, taking light and heat with them. It is a similar phenomenon that makes the moon shine; after sunlight hits its surface, rays of light are reflected off it and travel all the way to the surface of Earth, where we see them and see the ‘light’ from the moon.
In the case of greenhouse gases, molecules of gas trapped in the atmosphere absorb the light and heat that is reflected off the Earth and trap the heat energy that would otherwise go back into space. This trapped energy heats up the atmosphere. When that energy is released from the molecules, it rebounds back into the Earth’s atmosphere and is released within our atmosphere. Greenhouse gases in the atmosphere keep our planet warm with the heat they retain from the sun.
The problem arises when too many molecules of greenhouse gases accumulate in the atmosphere. When we burn greenhouse gases, we send extra molecules of carbon dioxide, methane, and nitrous oxide up into the atmosphere, we upset the delicate balance of heat exchanged and retained on our planet. All the extra gases hold in too much heat energy from the sun; the excess energy is retained and rebounded onto Earth, raising temperatures globally and causing the countless effects of climate change that we all bear witness to today. Life on Earth is held in a delicate temporal balance, surviving only within a specific range; when we push temperatures outside of that range, we push life on Earth to the edge of survival. The rapid rise in global temperatures is not limited to our planet’s surfaces; in fact, most of the warming is absorbed by our oceans. The ocean absorbs more than 90% of our planet’s anthropogenically-induced excess heat.
The ocean can take on the bulk of this burden because water has a high heat capacity, almost 4 times that of the air. This means that if it takes 1 unit of heat energy to raise the air temperature by 1 degree, it will take 4 units of heat energy to raise the ocean temperature by the same 1 degree. The capacity for various substances to absorb and retain heat varies based on their molecular structure, chemical bonds, and density, and so the amount of heat needed to raise the temperature of a substance is unique to that substance. Water has strong hydrogen bonds between the hydrogen and oxygen molecules, as well as a unique angular structure that lets it retain a lot of heat energy without heating up itself.
The high heat capacity of water allows the ocean to act as a thermal regulator for our planet. Thanks to this property, anthropogenically released greenhouse gases have resulted in relatively little change in Earth’s global average temperatures. According to the International Union for Conservation of Nature, in the past 100 years, the average global sea surface temperature has only risen by only about 1°C, even though humans have increased the abundance of carbon dioxide in the atmosphere by 43%. The extraordinarily low change in ocean temperatures, when compared to the sheer amount of greenhouse gases we emit, is a testimony to our planet’s natural ability to self-regulate even when under extreme stress.
However, no system is invulnerable to change, and even 1°C of ocean warming can have a dramatic effect on the ecosystems and aquatic life that lives there. As you know, reef-building corals have already begun to suffer from the effects of anthropogenically-induced ocean warming. Rising ocean temperatures threaten the vitally important symbiotic relationship between corals and zooxanthellae and lead to coral bleaching.
Still, as Maurice Yonge observed, in cases of periodically high temperatures corals can recover from bleaching. The question must be asked: if corals have the capacity to recover from periodically high temperatures, if expulsion is not always permanent and zooxanthellae can repopulate their tissues once more, why then should we be concerned about climate change and coral reefs? Shouldn’t they be able to take care of themselves?
If heat was the only stressor affecting coral reefs, then perhaps they could recover on their own, evolution helping the best fighters become more heat-tolerant by preserving zooxanthellae with machinery that can withstand higher temperatures.
Unfortunately, nowadays coral reefs are fighting against more than just heat. In 2014 to 2017, unprecedented coral bleaching events occurred across the globe. Heat stress on coral reefs has increased successively since the 1980s, however, the mass bleaching observed over the three-year time period was unique in that it is believed to be the longest-lasting, most widespread, and most damaging global coral bleaching event. In 2015, the Earth experienced an El Niño event, a worldwide warming of sea surface temperatures caused by weaker winds travelling across the equator. Though warmer temperatures do correlate to more bleaching, in many places the sheer extremity of bleaching didn’t seem to correlate to the amount of heat experienced. For example, researchers studying reefs in the Farasan Banks in Saudi Arabia found that bleaching there was inconsistent with the heat stress and light stress the corals experienced at the time; though in other years, sea surface temperatures were hotter and experienced more direct sunlight than in 2015, for some reason bleaching in 2015 was greater than ever before.
If not heat, what could induce such mass bleaching? Careful research by teams of scientists across the globe all pointed towards a common answer: nutrients in the ocean.
Coral’s zooxanthellae symbionts require a careful balance of carbon, nitrogen, and phosphorus in a ratio of approximately 106:16:1 in order to function and reproduce. Nitrogen (and less commonly, phosphorus) is limited in the ocean, which limits zooxanthellae growth. Without this limiting nutrient, even if there is excess carbon or phosphorus available, zooxanthellae cannot use it as they don’t have enough nitrogen around to make cell walls and replicate DNA, vital steps in their reproductive process.
When primary producers like zooxanthellae die, their dead cells sink down to the bottom of the ocean and are incorporated into the sediment. The sediment below the water can hold more nitrogen and phosphorus than the water above it. Nitrogen and phosphorus are condensed, compacted, and stored here indefinitely.
Winds that blow around the planet and across the oceans can change the direction of ocean currents, pushing water up from the bottom of the ocean towards the surface, pushing nutrients in the sediment up along with it. This is called an upwelling of nutrients. These nutrients travel through the shallow waters to areas with life, like the coral reefs, and primary production growth, no longer limited by nitrogen, explodes.
More symbionts in coral tissues may sound like a good thing, however, under elevated temperatures, more zooxanthellae lead to more reactive oxygen species being produced, which makes corals more sensitive to temperature changes, and increases the prevalence of bleaching events, increasing coral susceptibility to death and disease.
Excess nutrients are the leading hypothesis as to why bleaching in 2015 was so extreme. The scientists studying corals in the Farasan Banks found that in previous years, bleaching occurred when water temperatures were warm, but nutrient concentrations were low. When nutrient concentrations were high, but waters weren’t particularly warm, bleaching events occurred. When nutrient concentrations were high and water was slightly warmer, mass bleaching events happened to an unprecedented extent even if the sea surface waters weren’t as warm as they were in other years. The evidence suggested that an overload of nutrients makes corals more sensitive to bleaching.
However, the El Niño event Earth experienced around 2015 doesn’t correlate to an upwelling of nutrients from the sediment. During El Niño, weaker winds travelling across the equator weaken the ocean currents that push cool water from the deep sea up towards the surface. When El Niño isn’t in effect, the upwelling of cold water spurred on by strong winds contributes to cooling the surface of the sea.
If the nutrient overload that led to mass coral bleaching wasn’t caused by upwelling, what was it caused by?
Unfortunately, upwelling of sediment from below the sea isn’t the only source through which nutrients make their way into the ocean: humans have been accidentally adding nutrients to the water for at least half a century. Excess nutrients may even seem tame compared to some of the other substances that we leach into the sea. Nutrients from our sewage discharge and agricultural runoff seep into coral habitats and lead to mass bleaching events like the ones observed in 2015.
Sewage from our septic systems is often discharged into lakes, rivers, and streams that run into the ocean. Human waste is ripe with nitrogen and phosphorus, zooxanthellae’s limiting nutrients. In addition, chemicals we absorb through medicines and food can be excreted through our waste, making their way into the ocean, where they can harm marine species. Some chemicals block hormone receptors in our bodies, like birth control. When concentrations of these chemicals make their way into our water, they can block hormone receptors in aquatic animals, interfering with reproductive function and organism development. If left untreated, nutrients and chemical constituents from our waste can pollute the oceans, kill fish, and sicken coral reefs.
Nutrients from agriculture make their way into the ocean as well – farmers spray nutrients like nitrogen and phosphorus onto agricultural fields in the form of fertilizers, to promote plant growth. When farmers spray excess nutrients on the fields, plants can’t absorb them, and they accumulate on the soil. Rainfall washes the extra nutrients off the fields and carries them to lower elevations – where they accumulate in large bodies of water, such as the ocean.
Since the Industrial Revolution, global anthropogenic inputs of reactive nitrogen (that is, nitrogen that can be used by microorganisms like zooxanthellae) have increased 13-fold. Nitrogen fertilizer made up half the total anthropogenic input in 2020. The increase may seem necessary to ensure grain production can keep up with the growing population. However, if this were the case, then all nitrogen in the fertilizer would be used by the crops, and runoff wouldn’t be an issue. In reality, most cereal crops require only 50% of the nitrogen fertilizer they are supplied. The rest washes off and is transported into lakes, rivers, and oceans.
While small-scale studies like the one on the Farasan Banks in Saudi Arabia proved a correlation between nutrient overload and coral bleaching, American scientist Mary Donovan wanted to figure out exactly how widespread and absolute the effects were. When studying the effects of stressors on aquatic life, one must consider that in the environment, no effect is isolated. Everything is interconnected, and the effects of one issue will definitely be impacted by the effects of another.
In 2015, Donovan and her team carried out their own observational experiments on a broad range of islands in the French Polynesia to study the interactions between stressors. They sampled more than 10 000 corals and recorded the effects of two variables: heat and nitrogen concentration. Their findings showed that when heat stress was low and concentrations of nitrogen were low, bleaching was low. When heat stress was low and nitrogen was high, bleaching was high. When heat stress was moderate, and nitrogen concentrations were moderate, bleaching was high, and when heat stress was high and nitrogen concentrations were high, unprecedented rates of bleaching occurred. The team noted that severity of bleaching that occurred in high nitrogen, low heat stress conditions was similar to bleaching that occurred at the highest level of heat stress.
Donovan’s findings suggested that the concentration of limiting nutrients like nitrogen has a great impact on coral’s sensitivity to temperature fluctuations. Increase the nutrient concentration, and corals’ ability to withstand heat stress greatly diminishes. Donovan and her team proved that rather than one or another in extremity, it is the combination of these two stressors that caused mass bleaching. However, in addition to increasing coral susceptibility to heat, excess nitrogen has another hidden effect that has not been well studied. Recent evidence shows that high nutrient conditions reduce coral calcification rates (the rate at which corals build their calcium-carbonate skeleton).
As you know, coral reefs are constantly building their own skeleton; using the nutrients provided from their symbiotic zooxanthellae counterparts, they secrete calcium carbonate from their tissues and build onto their skeleton, growing up towards the surface of the water. When corals die, microorganisms and algae eat their skeleton and make room for new growth in a process called bioerosion. A reef system is held in a careful balance of bioerosion (the breakdown of hard skeletons) and accretion (the growth of hard coral skeletons). Some bioerosion is necessary to make space for baby corals to grow, but the rate of new growth must be faster than the rate of bioerosion to ensure the reef continues to thrive. Unfortunately, nutrient pollution is reducing calcification rates, causing that delicate balance to shift towards more bioerosion and less accretion.
It seems counterintuitive. Corals creating calcium carbonate skeletons, a process called calcification, depends on the energy that zooxanthellae provide, and zooxanthellae use the nutrients carbon, nitrogen, and phosphorus, in a ratio of 106:16:1 to make that energy and reproduce. If zooxanthellae growth is limited by nitrogen, then adding nitrogen should increase zooxanthellae growth, increasing their production of energy and increasing the amount of energy provided to the corals, thereby increasing their rate of calcification. Right?
Unfortunately, the evidence shows this is not the case. Scientists do not yet fully understand why nutrient pollution would negatively affect calcification, but one theory suggests that increased nitrogen causes so much zooxanthellae growth that the plankton becomes limited by phosphorus instead of nitrogen. No longer limited by nitrogen, zooxanthellae populations grow larger and larger until they use up all the available phosphorus. Then, the zooxanthellae population cannot be maintained because of the limited availability of phosphorus; starved for phosphorus, zooxanthellae populations experience mass death, and once again their growth is restricted. All this time and energy spent reproducing, only to die, is energy that isn’t being passed on to the coral reefs. In this way the excess nutrients do not get passed on to the coral; instead, they end up as dead zooxanthellae cells in the coral’s polyp.
However, as I mentioned before, in nature, everything is interconnected, and no effect is isolated. Excess nutrients aren’t the only factor affecting calcification rates; scientists acknowledge that ocean acidification also plays an important role. Anthropogenic CO2 emissions are not only leading to warmer ocean temperatures but are also causing the ocean to acidify.
Life in the ocean is all part of a balanced and thriving ecosystem that evolved to function in basic seawater. Basicity or acidity of a substance is determined by the number of hydrogen ions within it. We developed a way to measure the number of hydrogen ions in a substance, and we used that measurement to develop the pH scale, with a range from 1 to 14, which quantifies the acidity or basicity of a substance. If a substance has less hydrogen ions, it is more basic, and it has a higher pH (between 7 and 14). If a substance has more hydrogen ions, it is more acidic, and it has a lower pH (between 1 and 7). Since hydrogen ions are positively charged, and acidic substances have more hydrogen ions, acidic substances take on a positive charge, while basic substances take on a negative charge.
The natural pH of the ocean is 8.2 (slightly basic). For the last 2 million years in Earth’s history, the water of the ocean, measured by its pH, has remained relatively constant. Now, though, the ocean is acidifying faster than ever before. Since the Industrial Revolution in the mid-19th century, ocean acidity has increased by 25%. That correlates to a 0.1 drop on the pH scale. This may not seem like much, but a 0.1 decrease on the pH scale represents ten times more acidity. Let me repeat that: thanks to anthropogenic influences, in the last 150 years the ocean has become 10 times more acidic than it remained for the 2 million years prior.
Concentrations of carbon are naturally cycled through the Earth’s systems, contributing to vital processes of life at all levels as they change state and chemical composition. However, during the Industrial Revolution, we started burning fossil fuels, wood, and oil and harvesting energy from the massive reservoirs of carbon within them. In doing so, we upset the balance of storage and release within the carbon cycle, releasing far more carbon into the atmosphere than the land or the ocean has the ability to take up, sequester, and store.
The excess carbon dioxide in the atmosphere from burning fossil fuels not only contributes to global warming but causes ocean acidification as well. Carbon dioxide from the atmosphere dissolves into the seawater, where water and carbon dioxide combine to form carbonic acid. Carbonic acid is a bonded chemical made up of two hydrogen ions, a carbon atom, and three oxygen atoms.
The chemical composition of the water, measured by its pH, determines if carbonic acid will deprotonate (lose a hydrogen ion) and turn into bicarbonate, a chemical compound made up of only one hydrogen ion, one carbon atom, and three oxygen atoms (it differs from carbonate in that it doesn’t have two hydrogen ions). Hydrogen ions are positively charged. When the ocean is more basic it has a negative charge, and carbonic acid will deprotonate (lose a proton, which is in this case a hydrogen ion) into bicarbonate. The positively charged hydrogen ion, attracted to the negatively charged water, will separate from the rest of the compound. At a pH of 8.34, 98% of carbonic acid will dissociate into bicarbonate.
If the pH of the water is extremely basic (a pH of 8.5 – 10) a second deprotonation occurs, and the single remaining hydrogen ion is separated from the bicarbonate, resulting in carbonate, a compound made up of one carbon atom and three oxygen atoms. Carbonate is devoid of all hydrogen ions – it only forms when the water is so basic, so negatively charged, that it pulls both positive hydrogen ions away.
The pH of the ocean is not uniform, so carbonate exists at the normal ocean pH of 8.2, but it makes up only 9% of the carbon chemical compounds that exist in the ocean. Since carbonate has easy-to-access carbon and none of the not-so-useful hydrogen ions, this chemical compound is the most important one that corals use to build their calcium carbonate skeletons.
However, ocean acidification is decreasing the amount of carbonate available in the water – if the water doesn’t have a strong negative charge, the hydrogen ions in bicarbonate don’t experience a strong enough pull to break away. Without carbonate, coral reefs can’t build strong, sturdy skeletons.
Ocean acidification in coral reef environments doesn’t just debilitate their carbonate structure production; it also invites invasive species to the reef, including the coral-eating Crown of Thorns starfish. The Crown of Thorns starfish, named for their needle-sharp spines that cover their 5 arms and body, feast on encrusting algae, sea sponges, and stony corals. When ocean acidification increases, the nutritional value of aquatic plants fluctuates, and marine organisms need to alter their diet to accommodate the change. These animals are forced to consume a higher density of less-nutritious food to compensate.
Ocean acidification, along with nutrient pollution and warmer temperatures, reduce corals’ defense mechanisms designed to protect them from predators. Ocean acidification also affects the desired diet of Crown of Thorns starfish, forcing them to eat more. The starfish, drawn to the reefs by the excess algae on the corals’ ectodermis and the lowered structural defenses against them, feed on algae and coral tissue itself, eating more than ever before to compensate for changing ocean chemistry making their food less nutritious.
When Crown of Thorn Starfish break down the reefs, they expose previously shielded areas to the open ocean, robbing many animals of their safe homes and inviting predators to the scene. Predators arrive, and consume the smaller species, leading to lower population density in the reef and lower rates of reproduction. Eventually, no small fish are left for the larger organisms to eat. The food web that once thrived in coral reefs collapses, and the vibrant ecosystem is no longer functional.
Ocean acidification models predict that if we continue to burn fossil fuels without change, calcification on reefs worldwide will decrease by 156% by 2100. If we neutralize emissions by 2100, calcification will decline by 149%. The results from these models are grim; they predict that in 2100, no stony corals will continue to create their calcium carbonate skeletons if emissions continue as normal.
However, reduced rates of calcification can’t be seen with the naked eye, and corals today, though threatened, are still able to survive and build their skeletons. The current pH of the ocean is still within a range in which corals can survive. When I swam through the reefs, I still saw vibrant fish, beautiful branching polyps, and a diverse ecosystem of life.
Unfortunately, there is another stressor that has devastating impacts on individual coral reefs. This stressor can kill them instantly, rendering all hope for reef recovery impossible without external intervention.
After my adventures on the reef, I wanted to talk about scuba diving to anyone who would listen. I asked my instructor thousands of questions about his experiences diving around the world. One instance he described stood out in my mind.
“In India,” he said, “We visited dozens of reefs. Once, just for variety, after seeing so many thriving, vibrant ecosystems, our guide took us to a rise that had been decimated by blast fishing.
“We swam down to the site, and on the ocean floor I saw a wasteland; everything was reduced to rubble, the branches of corals piled up in pieces on the ocean floor. It was like someone had created a landfill under the sea. The water in front of us was clear, empty, devoid of life.”
His ability to paint a picture, combined with the emotion in his voice, filled me with dread. How could a diverse, strong ecosystem such as the reefs be cut down so completely?
The answer was already in his words, blast fishing. Blast fishing is an illegal practice wherein fishermen use dynamite to stun and kill fish – they drop a small bomb into the water, it explodes, and dead fish float up to the surface. The United Nations Fisheries and Agriculture Organization has long-standing campaigns against blast fishing and other unregulated methods, expending effort and resources into culling these destructive practices.
Blast fishing can be dangerous and deadly to the fishermen as well as the fish, blast-fishing is relatively efficient; it takes much less manpower, less resources, and yields more fish in less time than traditional methods.
In underdeveloped countries where poverty and unemployment rates are high, people often don’t have the luxury to care about coral reefs and dying ocean ecosystems. They worry about feeding their families and themselves, having a safe place to live, and staying alive. The persistence of blast fishing, even with its dangerous risks, is a testimony to their desperation; on her expeditions to India, marine scientist Juli Bernad heard high tales of a blast fisherman who lost an arm to the practice, yet continued to fish that way, until he blew off his other arm as well.
Police and government try to rein in the illegal activity; however, it can be difficult to do so when the participants are armed with bombs. The problem persists despite stiff fines and penalties, simply because of the effectiveness and efficiency of the practice.
Blast fishing is devastating for ocean ecosystems. When bombs are dropped onto coral reefs, The scattered remains of the reef litter the ocean floor for decades to come. There is no hope for reef recovery, without significant human intervention. On its own, the reef is finished once it has been blasted. Even if baby coral try to grow and repopulate, their small branches are crushed by the weight of the fragments of their dead parents, rolling around on the ocean floor, impervious to their own movement in the currents of the ocean.
However, even more sustainable fishing methods can be taken advantage of; any kind of overfishing can decrease the number of fish that feed on the coral’s algal layer, leading to a buildup of algae that harms the reefs. Researchers observing reefs in subtropical Japan found that in areas with high densities of fishermen, the number of algal-eating fish near the reefs were lower, and that reefs without these essential types of fish tend to experience an accumulation of algal growth.
How can we encourage fishermen to switch to less detrimental, but more time-consuming and costly fishing methods? How can we get them to prioritize the long-term rewards of preserving coral reefs that they themselves might not be around to see? Evidence shows that fishermen are well aware of the ecological impacts of overfishing and harmful fishing methods; they just don’t, or can’t afford, to care.
If corals only had to contend with overfishing, they might be able to defend themselves; reduced fish would cause a buildup of algae, but without nutrient pollution, maybe algae production wouldn’t be that great anyway. Unfortunately, the effects of overfishing compound onto the effects of nutrient pollution and climate change. Less fish and more nutrients leads to more algal growth, which increases coral susceptibility to rising temperatures.
How exactly does algal growth harm the corals? The answer lies within the coral microbiome. The coral microbiome refers to the colonies of microscopic organisms that live and thrive on the algal cover of a reef. A large body of research has quantified the variety of microorganisms that live on the mucus layer that covers the ectodermis of a healthy coral; these microorganisms have a symbiotic relationship with corals, helping to facilitate metabolism and protect corals from disease, while corals provide the microorganisms with a surface to grow on and resources to consume.
Dr. Nyssa Silbiger, a marine ecologist, sought to study how anthropogenic stressors affect the coral microbiome when she established her lab at the University of Hawaii. Dedicated to spending her life researching coral health, she spent years amassing a qualified team. In 2012, she undertook a research study to determine exactly how these anthropogenically induced stressors were affecting the microorganisms that live on the coral’s ectodermal layer.
The team got to work. Scientists dived down to the coral reefs found in the natural environment and built squared enclosures around them, within which they would carry out their experiment. They divided the selected area into 8 equal-sized plots. They applied slow-release fertilizer diffusers to four of the plots (called treatment plots) to simulate the effects of nutrient pollution, leaving the other four plots untouched to use as a control for comparison to natural environmental conditions. They also enclosed two of the treatment plots with an open-topped mesh wire, to reduce fish access to simulate overfishing. Then, they mapped the coral colonies in each enclosure, using rulers to accurately scale sizes. They left their setup, letting the nutrients slowly seep across the corals, letting the mesh wire show how the corals would react to an unusual lack of fish. Members from the team returned every week for 4 months to take pictures of each colony in each enclosure. Then, once they had collected all the photos, they used statistical analysis of the size differences to score tissue growth and loss.
Their analysis provided predictable results; the plots treated with nutrients and restricted from fish access experienced greater algae cover on the corals, but this wasn’t enough for Silbiger and her team. They wanted to know exactly how increased algal cover caused by nutrient pollution and overfishing was affecting the corals’ microbiome. hey took DNA samples from the surface of the coral colonies as well, every month, for the duration of the experiment.
Through analyzing the microorganisms, Silbiger and her team found that increased algal cover and elevated temperature (from natural variations) suppressed the typical microorganisms that dominated the microbiome of a healthy coral. These conditions promoted the bloom of other microorganisms, including pathogens, which are microorganisms that lay dormant for long periods of time, waiting until their host’s (the coral’s) immune system is weakened to attack. These pathogenic microorganisms, usually suppressed by the colonies that thrive on healthy corals, took their chance to multiply rapidly when excess algal cover and high temperatures affected reef health.
The scientists used statistical analyses to determine how accurately the dominant microorganisms on coral’s surfaces could be predicted from external conditions such as algal content and sea surface temperature. They found that coral microbiomes could be predicted with 78.5% accuracy based on these conditions. From their photos and in-field observations of the experiment, Silbiger’s team found that increased algal cover induced disease and mortality. The coral tissues in the untouched control plots grew by 36%, while the other three plots, affected by nutrient pollution and lack of fish experienced tissue losses averaging around 30%.
In the open plots treated with nutrients but not restricted fish access, they found that coral mortality caused by nutrient pollution changed the influx of important consumers to the reefs. Parrotfish (a colourful fish that enjoys crunching on coral) predation resulted in tissue loss in only 7% of the predated corals and never caused any deaths in open plots unaffected by nutrients. In the open plots treated with nutrients, 92% of the coral species bitten by parrotfishes lost tissue and 62% eventually died. Increased algal growth, caused by excess nutrients, made orals irresistible to parrotfish.
Dr. Silbiger and her team found that all the factors they assessed, including elevated temperature, simulated nutrient overload, and simulated overfishing, led to significant microbiome disruption, blooms of pathogens, and an overall long-term increase in coral disease, tissue loss, and mortality – when they happened simultaneously. None of these stressors on their own had a fraction of the negative effect that two or more of the stressors had when corals experienced them in conjunction.
The team’s DNA analysis led to the conclusion that excess nutrients and contact with algae destabilized the microbiome of corals. Their conclusions sparked a connection. In a very similar manner, these stressors also destabilize another well studied microbiome – our own.
Have you ever been outside, in the dead of summer, in sweltering heat? If you have, you were probably warned about heatstroke. Humans, like corals, cannot be exposed to elevated temperatures outside of our comfort range without getting sick.
Have you ever left food in the fridge for too long, and found it neglected, forgotten, and covered in mold? You know not to eat that food, or you’ll get sick – when exposed to fungi and pathogenic microorganisms, humans get sick too.
Now, do you remember the last time you got sick? Maybe you stayed up late too many nights in a row, binge-watching TV shows. Or maybe you ate junk food every day while you were on vacation. Maybe you were super stressed about an exam.
Or maybe, usually, it was all three – when you sleep badly, one or two nights, you are unlikely to get sick. Your body can function with a little impairment, holding off viruses and keeping you healthy. When you eat unhealthy food, for a little while, or interspersed with healthy food, your body can make do with what you give it, capitalizing on the resources it has to do its job and keep you healthy and functional. The human body is a remarkably balanced system, that has a great capacity to fend off disease even when it is slightly impaired.
The problem arises when factors start to compound – you’re stressed about a test, so you stay up late at night and don’t sleep well. You eat poorly. Suddenly, all those factors add up, and without healthy food, and rest, your body is no longer able to keep up with its never-ending fight against disease. The microbial pathogens strike, you get sick, and you have to eat well, rest, and wait until your body regains its strength.
Coral reefs work in a very similar way – like humans, they have a remarkable ability to self-regulate, and can withstand stressors with relatively few negative effects. The problem arises when multiple stressors, like elevated temperatures, nutrient pollution, ocean acidification, and overfishing overlap, and the corals, no matter how effective their systems are, just cannot keep up. The difference is, when we get sick, we eliminate the stressors from our life until we recover. In the case of coral reefs, the stressors aren’t going away. They are only getting worse.
It is estimated that one-third of reef-building corals worldwide face elevated extinction risk because of anthropogenic factors. Warming trends continue to dominate the global ocean; nutrient concentrations will increase unless we change our agricultural and wastewater practices, and thanks to carbon emissions the pH of the ocean is ticking its way down the scale with every passing day. Unless we change our influence on the planet, the compounding effects of our pollution will increase coral mortality until there are no corals left to die.
If corals were faced with just one threat, perhaps they could fight it themselves, growing stronger and evolving to adapt to the changing world. Unfortunately, coral reefs are being bombarded by human-created threats; global warming, nutrient pollution, ocean acidification, overfishing, and all the effects that precipitate from those problems, as well. To survive in the face of all these stressors, like global warming, nutrient pollution, overfishing, and ocean acidification, the corals will need our help.
If we can reduce or eliminate even one of these problems, we can give coral reefs and the 4 000 marine species that depend on them a fighting chance at survival. If we cull emissions and reduce global warming, we can keep the ocean temperature within the range that zooxanthellae can work in – even if there were still periodically high temperatures during which bleaching events may occur, we know that the coral reefs can recover and the zooxanthellae within them can repopulate. If nutrient pollution makes corals more susceptible to bleaching, then by reducing the amount of fertilizer and sewage that enters the ocean, we can significantly improve coral’s resilience and ability to adapt to changing temperatures. If we reduce emissions, we can reduce the acidification of the ocean, and corals will continue to build the calcium carbonate skeletons up towards the surface of the water, growing towards safer waters and becoming stronger in the face of other threats. If we can prevent illegal and overfishing, even if nutrient pollution leads to excess algal growth on corals, we can maintain the populations of grazing fish that eat the algae and prevent algal overgrowth.
Between climate change, nutrient pollution, ocean acidification, and illegal and overfishing, coral reefs are in for the fight of their lives. Addressing even just one of these issues gives coral reefs hope. Addressing none is a death sentence. Luckily, there are more than a few passionate individuals, from activists to scientists to engineers to CEOs, who are dedicated to helping them survive in this changing world. Restoration efforts worldwide are proving successful; not only that, but successful restoration efforts attract scientists, tourists, and attention to the issue that otherwise may have gone unnoticed. In Sulawesi, Indonesia, successful reef restoration has inspired other nearby islands to covet their own efforts. “It’s the power of imagery,” said Richard Vevers, who worked on the restoration project and is the founder and CEO of The Ocean Agency, a conservation non-profit. “The big difference is that before the restoration, none of us talked about the reef. It wasn’t a conversation we had. Now, we are talking about it.”
Scientists are hard at work analyzing and preserving the DNA of strong corals suited to survive in a hotter world. These corals will become the future; they have the traits to survive even the strongest heat waves. They are like lifeboats that leave a sinking ship; most won’t survive, but if even a few will, then they can go on to repopulate and keep coral reefs alive. However, if the lifeboats are on fire, the chances of any corals surviving decreases greatly. This is what it will be like if corals have to suffer through not only global warming, but nutrient pollution, ocean acidification, and overfishing as well.
There is still a chance for coral reefs, but they need all the help they can get. Even small restoration efforts can make big changes – every coral reef holds different traits, and there’s no telling which traits will be well-suited to survive in the future. If we reduce nutrient pollution and overfishing, we can take some of the burden off of the reefs, allowing them to fight for themselves, helping us help them. Coral reefs want to survive; if we can give them a fighting chance, they’ll take it. If we protect as much of their diversity as we can, we can give coral reefs, a thriving ecosystem that has existed for millions of years, a chance to thrive for millions of years in the future.
The reefs I swam through in my scuba gear were beautiful arrays of life. I felt that I was witnessing a surreal gift from the Earth, an expression of majestic wonder than can’t be replicated by anything manmade. To think that all that beauty, all that magic, could turn into a wasteland of broken skeletal pieces, is heartbreaking. Coral reefs are one of the most beautiful places on Earth. I hope we can ensure they survive long after I’m gone, long after you’re gone, so that the generations that come after us can experience their beauty and awe as I did.
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Serena Around the World 